Apple guy John Gruber uncovers an undocumented feature in Leopard: the ability to link any Mail.app email message from any other Mac application using the message: link protocol. This feature is perfect for referencing a message in a to-do, or in your notes, and it's available in the new version of Gmail, too. When you open any message in the new Gmail, its unique URL shows up in the address bar. Just copy and paste it to any document or other webapp and you've got a permalink to an email for quick reference later.